1. Field of the Invention
The present invention relates to an optical data link dual wavelength coupler and, more particularly, to such a coupler which provides either bidirectional or unidirectional communication over a single optical fiber.
2. Description of the Prior Art
In optical communication systems, the need may arise to multiplex different wavelength signals onto a single fiber. For example, wavelength multiplexing is one method for achieving full bidirectional transmission on a single fiber. In its simplest form, a bidirectional system may comprise two stations, S1 and S2, which transmit information at wavelengths .lambda..sub.1 and .lambda..sub.2, respectively. Thus, station S1 needs a transmitter which operates at .lambda..sub.1 and a receiver which is tuned to wavelength .lambda..sub.2. Station S2, obviously, has the opposite requirements. Each station also needs a duplexing element to inject both wavelengths onto the single transmitting fiber. Although simple in theory, such an arrangement is cumbersome in implementation. For example, each station comprises a separate transmitter, receiver, and duplexer. Therefore, some sort of optical coupling must also be provided, for example, by using optical waveguides. Such coupling requires many expensive and time-consuming adjustments to achieve optimum alignment. Additionally, the optical losses attributed to this coupling, including attachment between the duplexer, fiber, transmitter and receiver, may degrade the overall performance of the station to an unacceptable level.
An alternative to this straight-forward implementation is disclosed in U.S. Pat. No. 4,592,619 issued to E. Weidel on Jun. 3, 1986. Weidel discloses an optical coupling element utilizing a variety of microoptic elements with spherical and plane surfaces for collimating, focusing and redirecting transmitted/received lightwaves. Although an improvement over the prior art, the Weidel arrangement utilizes at least one optical element which must be traversed twice by a received light signal. Further, Weidel is necessarily limited to providing coupling between both a transmitter and receiver to an optical fiber. However, there exist situations wherein a pair of transmitters, operating at different wavelengths, must be coupled over the same fiber (unidirectional transmitter).
Thus, a need remains in the prior art for a dual wavelength optical coupler which is robust in design and is capable of operating in either a bidirectional mode (transmitter and receiver) or unidirectional mode (two transmitters or two receivers).